Increasing production from oil and gas wells



menac ng-1e, 1 954 i T D S mnnnAjsmG recoveries -.-o;n. am)

GAS WELLS time 0. Bond, Crystal Lake, and Ggwrgc Q er hica BL, as i nors to The W n "Qhicego, ll-14a gcorporation at l s Ohl ' This invention relates to a new method for treating various types of siliceous formations or strata bearing oil; gas, water or other fluids, and whichconsist's in introducing an anhydrous hydrogen halide mixture, or introducing two or more anhydrous hydrogen halides successively or separately in an oil well for the purpose of caus-' ing the halide or halides to react with 'or dissolve the siliceous formation in such a manner-as to result'in an increase in the flow of fluids that can be obtained from the strata so treated.

Previous to this invention acidizin'g methods have been used to treat siliceous formations with varying degrees of success in'oilrecovery operations. Many wells on being acidized form troublesome water-in-oil type emulsions. Other wells, on being treated with acidsolutions direct- 1y, or with acid solutions formed in situ, experie'nce a decrease in production instead of the much desired increase. The acidization of sandstone, formations has presented a particularly difficult problem. Hydrochloric acid has been applied to sandstone formations, especially those formations which are clogged with various dc: posits, of wax,.parafii-nlike bodies and various metal carbonate precipitates.

Hydrofiuoric and fi'uoboric acids have been tried but leave much to be desired. The use of hydrofluoric'acld by anyone other than a skilled chemist or technician is hazardous due t the severe and persistent burns vvhichit causes when in contact with the skin of the operator. Another rather difiicult obstacle stands in the way of eflicient andeiiective application of hydrofluoric acid to oil wells, that is, following the reaction of hydrofluoric acid on silica to-form water and silicon tetrafluoride, the latter compound reacts with any water formed, or already'present.

' our invention. We have also found that a mixby using a mixture of a mineral acid and aqueous hydrofluoric acid in th'e tr'eatment. Patent No.

2325;695 has proposed the injection of aqueous hydrofluoric acid into the-siliceous formation,

allowing sufficient time. for the acid to'reacttherein, and then injecting sulilei-'ent quantity of aqueous hydrochloric acid to dissolve the gela. I

tino'us silica-fluorides, siliclc acids and fiuosilieic acids which may have formed. All of these previous methods, while. having some merit, U I have not satisfactorily eliminated the possibility a of plugging due to formation of insoluble o'onrpounds and the uncertainty of the final result.

We have now discovered a new method of treating siliceous-formations with certain I drous agents which i s'a substantial improvement over theseprior methods in that substantial increases in peimeability can be obtained with no interference due to the formation'of insolubles during. the reaction. We have found that in creased permeability without plugging can be attained through the employment of. anhydrous agents under proper temperature conditi'ons'in lieu of the aqueous solutions previc'n sly-used. We

contemplate the passage of an amount of an ydrous hydrogen chloride throug'h'the sandstone formation followedby the passage of anhydrous hydrogen fluoride therethr'ough as one form of ture of anhydrous hydrogen fluoride and a gas inert; to hydrogen fluoride under the conditions experienced is an excellent medium, for increasto yield hydrated silica and hydrofluosilicieacid."

These reactions may be stated somewhat as follows: I

2H2Fz+SiO2- 2H2O+S1F4 3SiF4+3H20- 2H2SiFs+H2SlO3 The resulting gel clogs the pores--01 the producing formation, thereby decreasing rather than increasing the now of fluid.

Various modifications of vhydrofluoric acid acidizing technique have been proposed. In I]. 8. Patent N 1,990,969 there is disclosed a method of using hydrofluoric acid for treating siliceousformations, the hydrofluoric acid being formed in situ by the interaction of an acid and a fluoride. 7

U. S. Patent No. 2,094,479 proposes to overcome the problem of the formation of insoluble toaction products of silicontetrafluoride and water ing the permeability. orgasificati'on of siliceous formations, especiallysandstone or-quartz formations, which have been previously treated with anhydrous hydrogen chloride. The method of our invention represents a departure from the old methods in that the products-of the reaction are maintained in the rapor'and/or gaseous state. 40

It is, therefore, a. fundamental object 0'1 our invention to provide a method of increasing the permeability of siliceous earth formations without attendant risk of plugging of the producing formation.

A second object of our invention is to achieve substantial increases in permeability of sandstone formations by treatment with a small amount of :anhydrous hydrogen chloride accompanied by treatment with anhydrous hydrogen fluoride.

A third object of th s invention is to provide a method of gasification of siliceous formations using anhydrous agents to thereby increase theirs? i permeability-and increase the flow of fluids there- I through under conditions-such that the products, of the reaction, especially the water formed in I become apparent from the cores becameplugged and no the reaction or the connate water present, will in the vapor stat to thereby overcome the fonnation or any gelatinous by produ'cts which hereto-fore have been troublesome.

Other objectsand advantages of our invention the more de ailed desc'r-iptionwhich follows. a

In practicing our invention wehave found that the presericeof small amounts of anhydrous hy- .dro'gejn chloride within the interstices of the sandstone formation prior to contactwith the silica-dissolving anhydrous hydrogen fluoride 'prevents the formation of insoluble and poreclogging hydrated silica or .hydrofiuosilicic acids while at the same time allowing the operator to In order tic-demonstrate as effect of water upon .the' reaction of hydrogen fluoride with sandstone acres. the same experiment as dcscribed above was repeated -=usin-g cores I had been. saturated with as much water vapor as possible under imposed conditions by passing nitrogen gas saturated with water'through' them take advantage of the fact that treatment with anhydrous acids brings about a g-asiiication o! ,the sandstone, which advantage is not experi:

enced by other methods and represents a distinct departure therefrom.

Another inherent advantage of our process which is not present in the now practiced methods of treating wells with liquid agents is that the .dificulties' caused by-the Jamin action and other capillary effects, which are always present when any liquid treating agents are used, are avoided.

To illustrate the improved'perrneabilities obtained by our process as applied to typical sandstone formations such as are encountered in an earth formation; the following experiments were conducted. v j

Cylindrical cores of Berca sandstone as obtained from Cleveland Quarries Company were cut into cylinders one inch in diameter and one inch in length. These cores, were ovens-dried at 220 F. for about 16 hours Then the cores were cooled in a desiccator and their permeability to nitrogen wasdeterrnined using. ordinary p'ernreability apparatus consisting of means for holding the core and means for forcing ,fiuids through the core at various pressures and temperatures.

at the desired constant temperature.

'. I An oil bath was used to maintain the core holder. Reductions in the weight of the hydrogen fluoride supply cylinder after eachrun were used to deter- Anhydrous HF treatment of dry cores until'the core reached a constant weight (runs Nos. 8, l and 1-1), era}; in run No. IZ-by immer' sion in water at 120 F. for a period of time suiiicient to saturate the core to the extent possible by this inethod. The cores of runs 9 .and '13 are only half saturated since the passage of nitrogen therethrough was stoppedat that point.

' The results are gh wn in Table II iollewin'gi TABLE]! AnhydrousJiF Midtown 1!ofsitrtqlrizztcrico'1'es' Temp" Time, Grams v '1. iron HE 3'" I 3:,

3 (3 3% v n n .200 2 50.0 .-1ao- 1' a 200 a 4.45 210 a0 0.0 250 2 4.5 m 0.0 -,o.o 250 2 4-.5 m 0.0 0.0 250 a 54.5 as o on "Po lni-tial-permmbllityinn illidsroyl. a "P finalpermcabilliyinmillldercys. 4 The tests shown in Table 11 again an unpredictability of results. =general. it can he said that the water has an' 'adverse effect upon the reactionot hydrogn fluoride on sandstone cores. There is noindication why disintegration coupled with increased permeability should occur in runs 8 and 9, whereupon a plugging effect was experienced in the balance or the runs despite the quantity of hydrogen fluoride usedor increase in temperature. 7

Having established the undesirable results cxw llow.

perienced withanhydrous hydrogen fluoride upon dry or wet cores, experiments were again duplicated using 0V 1'-l.-dIled .cores which had been treated with 2.7 grams of anhydrous hydrbgen chloride prior to the passage of anhydrous. I'

hydrogen fluoride therehrough. 'Thes'e-resul v are shown in the following Table V Temp., Time, Grams Plho. a F- :Hour-s HF PO Pl r:

7.4 2 9. 1 94 0. 0 0.00 150 6 '4. 5 152 1'9. 5 0.52 150 2 43. 2. 194 D 9 x0 2 4.5 183 O. 0 0- 250 2 90. 5 147 w u P -lnitlalpermeabilit n millldar "P i-fili l pcrm'cabllity in mums-2. 1

In runs Nos. 1 .2 and 4 of Table I, smaller amounts of hydrogen fluoride were used because further -quantities could be forced through the cores- TABLE 111 I HF treutmczitof wlecon'toi-ning core:

Temp. 'Iir drama P: N0. 0 F. Hours i HF PI. T1 i E 1 13 '12: 0.0 o. 100 1 -s 130 0.0 1 0. 100 I 1 0 120 0.0 -0. 200 2 122 123 w 200 2 s4- 105 200 a; .53 a a 240 2 127 60 In i a 240 2 us 7: n i

Qwnmn permeability in mlllldarcys. "P,=final permeability in millidarcys. Y

The information given in Table III establishes I the fact that the treating of sandstone cores with hydrogen chloride previous to treatment with hydrogen fiuoride,-g-reatly enhances the easewith which consistent-disintegration of these corescan be obtained. :.:It' is also apparent from Table III' From the results: shown in Table I, it can be I seen that thetreat'ment of dry cores'wi'th anhy- .drous hydrogen fluoride leads to very UHDYBdiCt-g v able results. There is no correlationbetwecn the L temperatures of treat, the time of' treat. the n'eight of hydrogen fluoride used and the ultimate increase or decrease in permeability attained.

that at temperatures below about 200' enhancement-is not experienced, while at 200.F.

or more, very consistent and repeated"lilislnteg a tion can be obtained;

. .In conducting these preliininary tests, it was J found that a great amount of heat is evolved in th reaction of hydrogen fluoride with cores. Thermocouples placedlnside the cores inaemoss I dicated a temperature n'se. of from 50 to 200 F.

within the first few minutes after contact with the hydrogen fluoride. Generally, the tempera-'- ture rise is g'reatest with rocks or cores of higher permeability. 'I-l-ius, the initial temperature be-'- low which plugging occurs, even with thehydro upon thea'mount of hydrogen fluoride used. For

" cores having a permeability. of l'to 200 md.,-

initial temperatures of about 160 F. are required to safeguard against plugging; For cores having a permeability of 500 to 1,000 md initial. temperatures as low as 100- 120 F. are sufilcient.

The number of grams of hydrogen fluoride shown in the above tables does not indicate the amount of agentthat reacted with the core-for the reason'that on those in which P1/Po was zero, a plugging effect was obtained, and. the

gen, chloride pretreatment, .dependssomewhat upon the. permeability of the rock or core and number of gramsof hydrogen fluoride shown was all that could be forced into the core. However,

in the runs in which an infinite increasein permeability was obtained, the cores often opened up sufiicijently to allow the passage of a stream of hydrogen fluoride therethrough and as a consequence, thenumber of grams of agent is high.

In its broader aspects,- our invention comprises a method of. increasing the permeability of siliceous formations by the injection of anhydrous hydrogen chloride and anhydrous hydrogen fluoride into the formation within the well, and maintains the temperature at the locus of reaction at the threshold temperatureof the formation-.- Although we prefer to-pr'ecede the H5 ingen chloride and hydrogen fluoride, as, for example, by using a mixture of these. agents in any desired proportions, provided there is sufficient hydrogen chloride present to overcome theformation of undesirable precipitates. The ratio of the amounts of hydrogen chloride to hydrogen fluoride will in general be about from 1:2 to 1:10.

In some instances it maybe desirable to pro:- heat the formation prior to the injection of the anhydrous agents. This can be accomplished by the injection of hot inert gases or inert fluids such as air, helium, nitrogen; carbon dioxide, oil, etc. The temperature of preheating may vary from 160 to 500 F., but preferably a temperature of about 200 F. is sufficient. I Some water is. formed in the reaction of th hydrogen fluoride with the siliceous formation,

- but this can. be vaporized at the temperature under which the process is carried out. The

jection with HCl injection, it has been .foundthat I with certain'form'ations good results can be obtained by the simultaneous injectionof the hydrothe treat, there are manyother acidic substances,

- drogen iodide.

' Many alternate methods of practicing our inwells of the order of -4,000- to 5,000 feet deep,- ,-I or: deeper, will not require any prior heating if j 5 .the rock has an initial permeability 01-100 millidarcys or more.

After the completionof thehreaction of the anhydrous agents with the formatioma flushing treatment ,may be used to remove theei uent reaction products. "Such flushing-tre tment may be conducted by any suitable inert flushing agent-1 It has been found that in certain instances natural gas may be used as a flushin agent sin'ceit I 1: It is most desirable to use is readily available. a flushing agent which will not cause the precipita'tion of insolubles or the neutralization of the eiiluent. Hydrocarbon. oil, nitrogen, air,

carbon dioxide or helium'may be injected as a.

final treatment of the formation. The flushing treatment continued until-all'products of the reaction are removed, which may take from one 1 to ten; hours.

In the preferred practice of our invention, the

pend on the initial permeability and the extent to whichthe hydrogen chloride forced into the. formation. The pressure of treatment may vary from 30 pounds er square inch to .3300 pounds per square inch. Following" mistreatment, which will consume anywhere from one. to four hours depending on the nature and initial permeability- -'of the formation, the formation is tre'atedwith from 4,000 to 8,000 pounds anhydrous hydrogen fluoride while th'e fonnation'still retains most of the heat previously impart-ed. 'The well is then closed in for a period otone to ten hours to allow the HF'to react with the formation, After the r anhydrous hydrogen-"fluoride treat, 'the folfn'iation 1 l l I is flushed with 10,000 to 40,000 cubic feet'of natural gas. 1

Since the effect of. the hydrogen chloride pretreat is to prevent the'precipitajtionof gelatinous v compounds of silicon during the initial stage of? bothgaseous and liquid, which could be used in place of the hydrogen chloride, such as, sulfur,

dioxide, sulfur trioxide, hydrogen bromide or hyvention will be apparent to-o'ne skilled inthe art without departing from the scopeof our inven presence of an inert diluent gas, preferably natural gas, inthe hydrogen fluoride introduced into the well will also help to keep the water from condensing because the partial pressure of the water is reduced.

Whether the-'sandstonewill plugupon contact with HF depends not so much on the initial temperature of they rock' as it does on the maximum temperature after the hydrogen fluoride comes 'into contact with the rock. This maximum temv perature is attained within n few minutes after HF injection. If the ro'chreaches a-ma'ximum temperature of 225 -250" F., a successful treat 7 follows, regardless'of the initial temperature. If a temperature of 225 F. isnot reached, a relatively impermeable rock is likely to plug. With quite permeable rocks, successful treatsare obtainedeven though-the initial temperature is the range from to til) . mi rtures of an anhydrous gas inert to tlon. These and other similar applications are considered within the purview of our invention. As an example, it may be expeditious to inject nitrogen or other inert gasunder pressure into 9 the formation, then, following the nitrogen injection, anhydrous hydrogen chloride is forced into the same formation with increased pressure. This last injection is followed by an injection of anhydroushydrogen fluoride at still-greater pressures and the gaseous eiiluent and products are from the well.

The pressure of treatment used will necessarily be suincient to overcome internal wellpressure I and force the agents into the formation. Insomeinstances, advantage may be gained by injecting then removed by releasing the pressure and 9.1-

lowing their reverse flow from the well. In treat I ing a well inthis manner, the nitrogen gas acts as a flushingagent since 'itis the last to come.

tained in these experiments.

: fluoride vunder theconditlons of treat, an- I hydrous hydrogen fluoride into the well. To substantiate, this, a series of experiments were conducted using both Berea sandstone cores and quartz cores. Using typical core treating apparatus, as in the previous experiments, weighed cylindrical cores or plugs-9,11 inch in diameter and 1 inch long, were dried at 220 F. tor about 16 hours and weighed after cooling in a'desiccaton permeability of the dried core was then. determined using nitrogen gas, then the core holder was placed in' a, constant temperature bath and- 2.7 grains oi anhydrous hydrogen chloride passed" through the core. After this treatment, a-mixture of anhydrous hydrogen fluoride and nitrogen was produced by passing nitrogen'gas through tagesare again emphasized by the follow ng perlmental-results' in wmcncore's'were a column of liquid hydrogen fluoride at a constant temperature, and with constant rate of in the core treating apparatus. After the treatrnentof each core, the column of liquid'l iydrogen fluoride was weighed to obtain the amount of hydrogen fluoride used. Thereafter, the core I was cooled to room temperature and its permeability to nitrogen determined. The core residue was then weighed'; Table IV gives the results ob- Iv EFF-N2 treatment of Berea sandstone cores Pep Maxi- Guis. Gms. cent mum. i I P, HF I HF in C018 1; P. Used Used Tempe.

gas

6 8.6 41.8 135 220 0 8 12.8 38.5 200 384 0 19 33,5 30.2 194 387 a as 511 47.5 235 307. o 47 133.8 42.3 275 391 m 42 63.7 I 39.6 265 323 an 72 56.7 55.8 290 329 an All of the cores22 to 28 were treated with- 2.7 grainsof anhydrous hydrochloric acid before treatment with the HF-Nz mixture. The maximum temperature of the cores was obtained by a thermocouple inserted in a hole drilled in the core. Thermocouple readings weretaken in 2 to minute intervals during the treatment. In general, the highest core temperature occurred after 5 to minutes reaction. It appears that a maximum core temperature of approximately 200 F. should be attained within afew'minutes after the hydrogen fluoride treatment has begun to avoid any likelihood of plugging. The temperature of the HF saturation was 0 0., and the HF-Nz mixture so produced was forced through .thecores at BOpounds per square inch gauge. 'Fon those cores (22 and '23) in which Io was found to be zero, there was a, complete plugging and no more than the stated amount of HF-N: mixture could be forced through the cores.

For the balance of the cores showing Io to beinfinity', the core was" volatilized and the HF-N: mixture flowedfreely therethrough so their temperatures toward the end of the experiment, be-

ing dependent on the rate of. flow of the HF-Nz, would have little meaning. For these reasons, only maximum temperatures after 5 to 10 minutes are shown.

Though we have alreadyshown the advantages to be gained in pretreating the cores with anhydrous hydrogen chloride prior to treatment with anhydrous hydrogen fiuoridathe same advanaaas aag flow. This mixture-was passed through the core with an HF-Nz mixture, as previously without prior saturation 'with'lahfiydr-ous gen chloride. The results are as Iollo'wsf From these results it is immediately apparent that the mere treatment of siliceous formations I t with anhydrous hydrogen fluoride .admixedwitli an inert fluid will not give consistent'results with- 1 out prior treatment-of the formations with anhydrous hydrogen chloride or similar anhydrous acidic agent.

. We have also found that our method of increasing the permeability 'ofiormation's of a siliceousnature applies to all types of siliceous formations. 1.. v

Mixtures of nitrogen andanhydrous hydrogen fluoride were passed'through rectangular quartz pieces which were previously washed with dilute I hydrochloric acid, washed with water and ,dried at C. These experiments definitely es'tab llshed the fact that quartz equally afiected by HF-Nz mixtures. Mixtures containing 38 to 51.

per cent hydrogen fluoride at '30 pounds per square inch volatilized'about the same amount of quartz as' will pure hydrogen fluoride at 1 ate mosphere pressure. It appears that the rateo! reaction on q rtz is approximately a function of the partial pressure of the hydrogen fluoride inthe'gas mixture. Our experience has taught that for the most consistent results, it is best to conduct the treat-.

ment oi siliceous. formations within a range of temperatures and pressures which will allow the hydrogen fiuroride and gresulti-ng 'or oo'nnate water and the silicon t'etrafluoride reaction product to-rezna-in in the vaporxandlor gaseous state. At most, a film of adsorbed liquid can .be-tolerated on the surface of thesand, for under conditions which permit the condensation of water ,or a

water-HF phase, the sandstone is often, though not always, plugged. Our experiments with solid quartzrectangular plates indicate. that the rate of reaction of silica with gaseous hydrogen fluoride decreases rapidly as the temperature is raised above the point at which a water-HF phase can exist.

necessary to apply some pressure to force the gaseous hydrogen fluoride into the interstices of the sandstone. The pressure required will depend upon the fluid pressure in the sandstone j and its permeablility. In general, pressures from 10 to 300 pounds per square inch may be necessary to force the anhydrous hydrogen fluor- I I ide and its gaseous products through the forma tlon under treatment. In proceeding under proper conditions of temperature and pressure,

all the products of. the reaction are maintained, .1, I in the gaseous phase and such effects as the Jamin action, plugging due to solid precipitates,

and by-passing, are avoided. I

The temperature limitations imposedby the When treating a well by our method, it will be t nature of this reaction depend on the reserd arrhoea.

voir temperature which in turn is dependent on the depth and pressure the well bore.

The reservoir temperature mayrang'e from 1 to 180 F., more or less. Since the vapor pressure of hydrogen fluoride is'between 50 to 70"pounds per square .inch'at these temperatures, it will be necessary to apply some heat .to raise the tem perature to ash-i811, as 200" to 260 F., when oper-' ating under atmospheric pressures. It may be necessary to heata portion 'of the formation nearthe bore hole before the introduction of the anhydrous hydrogen fluoride. to at'le'ast 200 R, and allow the heat of the vreaction to maintain the desired temperatures as the reagent attacks additional sandstone formation further from the bore hole. I

At 240 F., the vapor pressure of hydrogen,

fluoride is about 200 pounds per square inch. Therefore, a successful well ,treatmentcould be carried out with gaseous hydrogen fluoride at pressures not exceeding 200 pounds per square inch. The condensationof water vapor is tobe which the sm sh is preheated to a .tu-re of about 200 F. prior to the injection of said anhydrous acid material. 3 7 4;. The methodin accordaneewith 1 in which thelrcaction products are removedbyflushing said formation with an inert agent.

[5. The method in accordance with ,4 in which the inert flushing-agent is natural gas.

' -6.' 'Ihe method in accordance with 4 in which the inert agent'is heated toabout 200 F. I

'7. A metliod-of increasing the permeability ofsiliceous formations by the simultaneous injection therein of a mixture comprising (1), an m1- hydrous acid material selected from the group 7 consisting of hydrogenchlori'deihydrogcn iodide, 1 hydrogen bromide. sulfur dioxide, sulfur trl-i oxide and (2} -anhydrous hydrogen fluoride-the ratio of acid material to hydrd cn fluoride in I said mixture being within 1:2 to 1210, with surficient pressure to cause said-mixture to permeate said formation at a temperature above 260 F avoided'so when operating at formationternperature of about 240 F., the pressure could be as high as pounds per square inch before any condensation would occur, since water and hydrogen fluoride form a maximum boiling azeotrope.

At the beginning of the reaction, the unreacted hydrogen fluoride will dilute the water formed or- So, also,-, as the reaction proceeds, the silicon tetrafluoride present and reducelts partial pressure.

form-ed will act in'a similar manner. All temperature and pressure limitations are dependent on the original permeability'of the formation to be treated, as has been previously explained. The typical Berea sandstone used incur experiments showed onanalysis the following composition inwei'g-ht per cent:

i Per cent Mummum-nxirie 2,6 calcium oxidene 1.]; Magnesium oxide 0.2 Silica--. 4 92.9 Carbon dioxlde -.s e 3.2

The sandstone formations encountered in earth allowing said mixture to remain and react with.

said formation for at least about one hour, there by to maintain the reaction'produc-ts and eonnate water in-the g'aseous phase, and removing the re- Z I I I I action'products in the gaseous phase. ,f'

S. The method in .accordancewith claim '7 in which the anhydrous acid material is'lwdrogen chloride.

9. The method in accordance with 7 which the formation vis'pi eheated to a temperature of about 200 F. prior-to "the-injection of r said mixture.

10. The method in accordance with-claim 7 in which the reactionproducts are removed by flushing said'for'mation with an inert agent.

11-. The method in accordance with claim 10 in which the inert flushing agent is natural" gas.

12. The method accordance with claim 10.

in which the inert fiush ngagent is heated to fibOut 200F.

13. A method of overcoming the precipitation. of insolubles due' to the presence of conna-te water in siliceous formations during reaction processes to increase-"the permeability thereof, comprising bringing said formation to a temperature of at least 200 F. in the presence of an anhydrous acid material selected from the group (1) conoxlde with sufiicient pressure to cause said acid material to permeate and substantially saturate said formation, the amount of said acid being s'uflicient to prevent the precipitation of gelatinous compounds of silica, thereafter injecting anhydrous hydrogen fluoride into said formation under such conditions that the reaction tempera- 'ture ls-maintained at above200 F., the amount of said hydrogen fluoride being 2 to 10 times the amount of anhydrous acid, allowing said acid material and hydrogen fluoride to react with said formation for at least about one hour, thereby to maintain the reaction products and' 'connate.

water in the gaseous phase, and removing'the reaction products in the gaseous phase.

2. The method in accordance with claim 1 in which the anhydrous acid is hydrogen 1 chloride.

I. I; 3;. Thc method inaccordance with 1 in 15 sisting of hydrogen chloride, hydrogen iodide,

hydrogen bromide, sulfur dioxide, and sulfur H trioxide and in the presence of (2) anhydrou hydrogen fluoride, the ratio of the amount of acid material from group (I) to group '(;2) being within the range from .112 to 1:10, allowing said reagents to react with said formation under conditions that the reaction temperature is at least 200 R, and removing the reaction products in the gaseous phase.

14. The method in accordance with l3- 1 in which the formation is heated by the iniection of heated inert gases.

15. The method in accordance with cla'm 13* in which the formation is heated by injection 3 of heated inert liquids. 1

1s. The method 'in accordance with c1aiml3; l in which the time of reaction is from 1 to 4 hours.

17. The method in accordance with claim 131- I in which the materials. from group (1) and group (2) arejinjected simultaneously as a nurture in the prescribed ratios.

i 1 d i i 5. 1 ,v

18. The method in awnrdalce with mm; 11

in which the material from groupfl) is hydrogen chloride.

19'. The method in warn-fiance with claim 13 in which the reactiomproducts. are removed 9n completion of the by tin-action of a 1 inert flushing agent.

'20. The mem -am mrfiaxic e with. claim 13 in which the-material imm group (1) is hydmgen chloride.

Rofmfeqcm Giicd in-tho me .0! this Wteflt Human STATES iin'wm'rs 

1. A METHOD OF INCREASING THE PERMEABILITY OF SILICEOUS FORMATIONS BY INJECTION THEREIN OF AN ANHYDROUS ACID MATERIAL SELECTED FROM THE GROUP CONSISTING OF HYDROGEN CHLORIDE, HYDROGEN IODIDE, HYDROGEN BROMIDE, SULFUR DIOXIDE, AND SULFUR TRIOXIDE WITH SUFFICIENT PRESSURE TO CAUSE SAID ACID MATERIAL TO PERMEATE AND SUBSTANTIALLY SATURATE SAID FORMATION, THE AMOUNT OF SAID ACID BEING SUFFICIENT TO PREVENT THE PRECIPATION OF GELATINOUS COMPOUNDS OF SILICA, THEREAFTER INJECTING ANHYDROUS HYDROGEN FLUORIDE INTO SAID FORMATION UNDER SUCH CONDITIONS THAT THE REACTION TEMPERATURE IS MAINTAINED AT ABOVE 200* F., THE AMOUNT OF SAID HYDROGEN FLUORIDE BEING 2 TO 10 TIMES THE AMOUNT OF ANHYDROUS ACID, ALLOWING SAID ACID MATERIAL AND HYDROGEN FLUORIDE TO REACT WITH SAID FORMATION FOR AT LEAST ABOUT ONE HOUR, THEREBY TO MAINTAIN THE REACTION PRODUCTS AND CONNATE WATER IN THE GASEOUS PHASE, AND REMOVING THE REACTION PRODUCTS IN THE GASEOUS PHASE. 